  A mixture of light and heavy atoms is considered. We study the kinetics of
the light atoms, scattered by the heavy ones, the latter undergoing slow
diffusive motion. In three-dimensional space we claim the existence of a
crossover region (in energy), which separates the states of the light atoms
with fast diffusion and the states with slow diffusion; the latter is
determined by the dephasing time. For the two dimensional case we have a
transition between weak localization, observed when the dephasing length is
less than the localization length (calculated for static scatterers), and
strong localization observed in the opposite case.
