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We study the influence of a normal magnetic field on the 0.7 feature exhibited by quantum point contacts (QPCs). The magnetic field is used to induce the formation of edge states whose spin splitting and spatial separation can be varied directly via the applied field. By appropriate control of the gate voltage, the QPC can be configured so that its conductance is determined by the two spin-resolved edge states of the lowest Landau level, mimicking the two-channel picture that has been suggested in discussions of the 0.7 feature. Under these conditions, a clear 0.7 feature is only observed at weak magnetic fields, where any spin gap is small and the two edge states are strongly overlapping. A similar feature is also seen at high magnetic fields, but only once the temperature is increased such that the thermal energy is comparable to the size of the spin gap. The connection of these results to the processes that lead to the 0.7 feature is discussed.

We investigate the linear transport properties of quantum point contacts (QPCs) whose symmetry is deliberately broken in a controlled manner. The devices that we study consist of a conventional split-gate QPC, which is modified by the inclusion of an additional perturbing gate that is used to modulate the electron density on one side of the device. As the voltage applied to this 'finger gate' is varied, we observe several reproducible features below the last integer plateau, as well as strong modifications of the integer-plateau staircase. Self-consistent calculations, performed for the exact device structure utilized in experiment, suggest that these features are related to the ability of the finger gate to strongly disrupt the symmetry of the QPC, reducing the electron density significantly on one side of the device. We discuss these results within the context of recent models for many-body electron transport in QPCs.

We study the scaling properties of the conductance fluctuations due to quantum interference in linear quantum-dot arrays. The key finding of our study is of a common scaling behaviour of the fluctuations, according to which their amplitude varies in direct proportion to the total conductance of the system, independently of the size of the arrays. We argue that such behaviour is inconsistent with a classical scaling of incoherently coupled dots, but is reasonable for that expected when dephasing arises from the interaction of the quantum system with its external environment.

Evidence for dynamical tunneling is observed in studies of the transmission, and wave functions, of a soft-walled microwave cavity resonator. In contrast to previous work, we identify the conditions for dynamical tunneling by monitoring the evolution of the wave function phase as a function of energy, which allows us to detect the tunneling process even under conditions where its expected level splitting remains irresolvable.

Recent work on the role of decoherence in quantum physics has suggested that the quantum to classical decay is governed by a discrete set of pointer states, which are quite stable and uncoupled from other states in the system. We show that the conductance oscillations exhibited by open quantum dots are governed by a discrete set of stable quantum states which have the properties of the pointer states, and which are closely related to trapped classical orbits in the open dot. These states are essentially classical in nature, as evidenced by their energy level spacing, and their decay is apparently in the environment as opposed to within the dots.

We discuss the influence of the electron–electron interaction on transport properties of open quantum dot systems. Based on the idea of the Anderson model, we present interaction-induced temperature-dependent corrections to the conductance beyond the single-particle picture.

In this review, we discuss the results of recent experimental studies of the low-temperature electron dephasing time (τφ) in metal and semiconductor mesoscopic structures. A major focus of this review is on the use of weak localization, and other quantum-interference-related phenomena, to determine the value of τφ in systems of different dimensionality and with different levels of disorder. Significant attention is devoted to a discussion of three-dimensional metal films, in which dephasing is found to predominantly arise from the influence of electron–phonon (e–ph) scattering. Both the temperature and electron mean free path dependences of τφ that result from this scattering mechanism are found to be sensitive to the microscopic quality and degree of disorder in the sample. The results of these studies are compared with the predictions of recent theories for the e–ph interaction. We conclude that, in spite of progress in the theory for this scattering mechanism, our understanding of the e–ph interaction remains incomplete. We also discuss the origins of decoherence in low-diffusivity metal films, close to the metal–insulator transition, in which evidence for a crossover of the inelastic scattering, from e–ph to ‘critical’ electron–electron (e–e) scattering, is observed. Electron– electron scattering is also found to be the dominant source of dephasing in experimental studies of semiconductor quantum wires, in which the effects of both large- and small-energy-transfer scattering must be taken into account. The latter, Nyquist, mechanism is the stronger effect at a few kelvins, and may be viewed as arising from fluctuations in the electromagnetic background, generated by the thermal motion of electrons. At higher temperatures, however, a crossover to inelastic e–e scattering typically occurs; and evidence for this large-energy-transfer process has been found at temperatures as high as 30 K. Electron–electron interactions are also thought to play an important role in dephasing in ballistic quantum dots, and the results of recent experiments in this area are reviewed. A common feature of experiments, in both dirty metals 3 Authors to whom any correspondence should be addressed.

Recent advances in semiconductor microprocessing technology now allow the realization of sub-micron sized quantum dots, which are quasi-zero-dimensional devices in which current flow is confined on length scales approaching the Fermi wavelength of the electrons. The influence of disorder is thought to be strongly suppressed in these devices, so that electrons propagate while mainly undergoing large-angle scattering at the walls of the dot. At sufficiently low temperatures, electron phase coherence is maintained over long distances and coherent interference of electrons becomes an important process in determining the electrical behaviour of the dots. In this review, we focus on a number of issues revealed by recent experimental studies of open dots, such as fractal magneto-conductance fluctuations, wavefunction scarring due to selectively excited periodic orbits, and novel `ratchet' behaviour in non-equilibrium studies.

We report geometry-induced fractal behaviour in the low-field magneto-conductance fluctuations of a mesoscopic semiconductor billiard. Such fractal behaviour was recently predicted to be induced by the mixed (chaotic/regular) phase space generated by the soft-walled billiard potential, and our results constitute a possible experimental observation of the infinite hierarchical nature of this mixed phase space. Preliminary investigations of the effects of temperature and gate bias, which directly control the electron coherence and billiard potential profile, are presented.

We perform numerical simulations of the quantum mechanical transport and corresponding wave-functions in open, square quantum dots. The effects of inelastic scattering are introduced phenomenologically. Examining the root mean square amplitude of the conductance fluctuations, we find they decay exponentially with ( being the inelastic scattering time), in good agreement with the results of recent experiments. In previous work, we have found the wave-functions of such dots should be `scarred'. We examine the robustness of this scarring effect to the phase-breaking process.

The authors have measured the magnetoresistance of one-dimensional GaAs heterojunction wires at temperatures down to 0.04 K and in magnetic fields of up to 1.2 T. At low magnetic fields they observe universal conductance fluctuations with a temperature dependence consistent with a temperature independent phase breaking length. This unexpected result is not predicted by theory and they speculate that it may be related to the breakdown of diffusive motion.

We investigate edge state transmission in quantum point contacts (QPCs) in the fractional quantum-Hall regime, finding behavior reminiscent of a metal-insulator transition. The transition is suggested by an unusual behavior of the differential conductance in the fractional-quantum-Hall regime, and by the presence of a fixed point and universal scaling in the temperature dependence of the linear conductance. Noting that the 0.7 feature evolves continuously into a last fractional plateau at high magnetic fields, we suggest that this still unresolved feature may itself be viewed as a manifestation of a local, microscopic, metal-insulator transition.

We investigate the behavior of interacting one-dimensional systems using linear (close to equilibrium) and non-linear transport measurements of split-gate quantum wires of varying channel length. Our measurements reveal a remarkable resonance effect in the differential conductance, which exhibits a pronounced peak, for a narrow range of source–drain voltage, at the transition from tunneling to open transport. This peak becomes more pronounced with increase of channel length, but is rapidly suppressed by increase of temperature or (in-plane) magnetic field. We believe that these unique features may arise from the dependence of transport on the electron density of states, and suggest a phenomenological model to account for this transport behavior.

For many years now, there has been ongoing interest in the manifestations of many body phenomena in the conductance of strongly confined, one-dimensional (1D) electron systems. One important aspect of this research has centered on the study of the so-called '0.7 feature' in the low-temperature conductance of 1D conductors known as quantum points (QPCs). There have been numerous reports in the literature suggesting that the 0.7 feature should be related to some kind of spontaneous spin polarization in the QPCs, which persists even at zero magnetic field. In this report, we review the results of our recent work on this problem, in which we make use of coupled QPCs to probe the properties of transport very close to pinch off. We observe a resonant interaction between two QPCs whenever one of them pinches off, which we believe is associated with the binding of a single spin to the QPC that is pinching off. A phenomenological theoretical model is developed that relates the observed resonance to a tunnel-induced correlation that arises from the interaction between a presumed bound spin on one QPC and conducting states in the other. Building on these ideas, we use this measurement technique to probe the microscopic properties of the bound spin, finding it to be robustly confined and to show a Zeeman splitting in a magnetic field. The spin binding occurs for stronger gate confinement than the 0.7 feature, and we therefore suggest an alternative scenario for understanding the formation of this feature. In this, one considers the evolution of the self-consistent bound state as the gate potential is weakened from pinch off to allow for electron transmission through the QPC. The suggestion of this work is that a QPC may serve as a naturally formed single-spin system with electrical readout, a finding that may be useful for the development of future generations of single-spin electronics.

For over 25 years, studies on one-dimensional transport phenomena in semiconductors have been carried out: first in silicon, then in gallium arsenide and presently in numerous other materials. These studies have provided an extremely effective means to investigate a broad variety of mesoscopic and nanoelectronic phenomena. In the ballistic (point contact) regime these structures show excellent examples of quantized conductance in integer units of the fundamental quantum of conductance, 2e2/h. For a long time this has been understood to arise from the almost perfect transmission of one-dimensional subbands. On the other hand, the origin of the 0.7 structure, an additional plateau-like feature that occurs ubiquitously at a conductance value close to 0.7 × 2e2/h remains undetermined. Interest in the 0.7 structure, which is now approaching its twelfth year, is helping to raise awareness of the interesting effects that can be observed in semiconductor nanostructures, and also the versatility of these devices. The 0.7 structure has been apparent from the earliest days of one-dimensionality and its remarkable persistence in both published and unpublished data has led to detailed investigations and the finding that it is clearly related to electron spin. It has now been observed in a variety of one-dimensional systems and investigations have widened to include spin effects and the consequences of electron interaction in general. The effect has also stimulated theorists with many explanations being proposed. This has taken the theory into new areas, which in turn has led to new experiments. When a special issue devoted to the 0.7 structure was first proposed, the level of interest that it would generate was unclear. However, the scientific community's enthusiasm for the idea, and significant interest from authors, were extremely gratifying. We must express our thanks to all of the authors for their cooperation and help in preparing their articles on time. It is hoped that this special issue will help convey, to a wide audience, the opportunities that modern semiconductor technology brings to the study of nanostructures and the flexibility which exceeds by far that achieved in other systems. These systems are outstanding laboratories of quantum solid state physics and their full potential is yet to be realised. We are grateful to the authors for their contributions and to IOP Publishing for their efficiency and unfailing courtesy during this collaboration.

We explore the use of the split-gate method to implement coupled-electron waveguides (CEWs), for possible application as a scalable qubit in quantum computing. Electron switching in these structures is found to be strongly influenced by structural asymmetry, which appears to arise quite naturally when using the split-gate method to implement complicated nanostructures. The consequences of the asymmetry are shown to depend strongly on the manner in which the input and output ports of the device are configured, and include unwanted decoherence due to geometry-induced scattering of electrons into regions outside of the classical current path. An approach to eliminating the influence of this asymmetry, to allow the realization of a 'balanced qubit', is also explored.