  We extend the Landauer-B\"uttiker formalism in order to accommodate both
unitary and self-adjoint operators which are not bounded from below. We also
prove that the pure point and singular continuous subspaces of the decoupled
Hamiltonian do not contribute to the steady current. One of the physical
applications is a stationary charge current formula for a system with four
pseudo-relativistic semi-infinite leads and with an inner sample which is
described by a Schr\"odinger operator defined on a bounded interval with
dissipative boundary conditions. Another application is a current formula for
electrons described by a one dimensional Dirac operator; here the system
consists of two semi-infinite leads coupled through a point interaction at
zero.
