Problem of light diffusion in strongly scattering media

Recently, in connection with the problem of Anderson localization of light, a new physical mechanism was considered for the decrease of the diffusion coefficient, in which it is supposed that the velocity of energy transport, which appears in the diffusion constant D= @23E l tr /3, may be much smaller than the phase velocity. Using an exact definition of the diffusion coefficient and generalized Ward identity, we, however, show that in the low-density approximation υ E coincides with the phase velocity