  The Wigner-function formalism is a well known approach to model charge
transport in semiconductor nanodevices. Primary goal of the present article is
to point out and explain intrinsic limitations of the conventional
quantum-device modeling based on such Wigner-function paradigm, providing a
definite answer to open questions related to the application of the
conventional spatial boundary-condition scheme to the Wigner transport
equation. Our analysis shows that (i) in the absence of energy dissipation
(coherent limit) the solution of the Wigner equation equipped with given
boundary conditions is not unique, and (ii) when decoherence/dissipation
phenomena are taken into account via a relaxation-time approximation the
solution, although unique, is not necessarily a physical Wigner function.
