  In designing and optimizing new-generation nanomaterials and related quantum
devices, dissipation versus decoherence phenomena are often accounted for via
local scattering models, such as relaxation-time and Boltzmann-like schemes.
Here we show that the use of such local scattering approaches within the
Wigner-function formalism may lead to unphysical results, namely anomalous
suppression of intersubband relaxation, incorrect thermalization dynamics, and
violation of probability-density positivity. Furthermore, we propose a
quantum-mechanical generalization of relaxation-time and Boltzmann-like models,
resulting in nonlocal scattering superoperators that enable one to overcome
such limitations.
