  We consider a quantum device contained in an interval in the context of the
Weyl-Wigner formalism. This approach was originally suggested by Frensley, and
is known to be plagued with several problems, such as non-physical and
non-unique solutions. We show that some of these problems may be avoided if one
writes the correct dynamical equation. This requires the (non-local) influence
of the potential outside of the device and the inclusion of singular boundary
potentials. We also discuss the problem of imposing boundary conditions on the
Wigner function that mimic the effect of the external environment. We argue
that these conditions have to be chosen with extreme care, as they may
otherwise lead to non-physical solutions.
