  A general density-matrix formulation of quantum-transport phenomena in
semiconductor nanostructures is presented. More specifically, contrary to the
conventional single-particle correlation expansion, we shall investigate
separately the effects of the adiabatic or Markov limit and of the reduction
procedure. Our fully operatorial approach allows us to better identify the
general properties of the scattering superoperators entering our effective
quantum-transport theory at various description levels, e.g., N
electrons-plus-quasiparticles, N electrons only, and single-particle picture.
In addition to coherent transport phenomena characterizing the transient
response of the system, the proposed theoretical description allows to study
scattering induced phase coherence in steady-state conditions. As prototypical
example, we shall investigate polaronic effects in strongly biased
semiconductor superlattices.
