  Quite a many electron transport problems in condensed matter physics are
analyzed with a quasiparticle Boltzmann equation. For sufficiently slowly
varying weak external potentials it can be derived from the basic equations of
quantum kinetics, provided quasiparticles can be defined and lead to a pole in
the quantum-mechanical propagators. The derivation breaks down, however, in the
vicinity of an interface which constitutes an abrupt strong perturbation of the
system. In this contribution we discuss in a tutorial manner a particular
technique to systematically derive, for a planar, nonideal interface, matching
conditions for the quasi-particle Boltzmann equation. The technique is based on
pseudizing the transport problem by two auxiliary interface-free systems and
matching Green functions at the interface. Provided quasiparticles exist in the
auxiliary systems, the framework can be put onto the semiclassical level and
the desired boundary conditions result. For ideal interfaces, the conditions
can be guessed from flux conservation, but for complex interfaces this is no
longer the case. The technique presented in this work is geared towards such
interfaces.
