  A new density matrix and corresponding quantum kinetic equations are
introduced for fermions undergoing coherent evolution either in time (coherent
particle production) or in space (quantum reflection). A central element in our
derivation is finding new spectral solutions for the 2-point Green's functions
written in the Wigner representation, that are carrying the information of the
quantum coherence. Physically observable density matrix is then defined from
the bare singular 2-point function by convoluting it with the extrenous
information about the state of the system. The formalism is shown to reproduce
familiar results from the Dirac equation approach, like Klein problem and
nonlocal reflection from a mass wall. The notion of the particle number in the
presence of quantum coherence is shown to be particularily transparent in the
current picture. We extend the formalism to the case of mixing fields and show
how the usual flavour mixing and oscillation of neutrinos emerges again from a
singular shell structure. Finally, we show how the formalism can be extended to
include decohering interactions.
