  In view of its local character, the semiclassical or Boltzmann theory is
intrinsically unable to describe transport phenomena on ultrashort space and
time scales, and to this purpose genuine quantum-transport approaches are
imperative. By employing a density-matrix simulation strategy recently
proposed, we shall demonstrate its power and flexibility in describing
quantum-diffusion phenomena in nanoscale semiconductors. In particular, as for
the case of carrier-carrier relaxation in photoexcited semiconductors, our
analysis will show the failure of simplified dephasing models in describing
phonon-induced scattering non-locality, pointing out that such limitation is
particularly severe for the case of quasielastic dissipation processes.
