  We generalize a Landauer-type formula, using a real$\otimes$spin-space Green
function technique, to treat spin-dependent transport in quantum-coherent
conductors attached to two ferromagnetic contacts. The formalism is employed to
study the properties of components of an exact zero-temperature conductance
matrix ${\bf G}$, as well as their mesoscopic fluctuations, describing
injection and detection of a spin-polarized current in a two-dimensional system
where electrons exhibit an interplay between Rashba spin-orbit (SO) coupling
and phase-coherent propagation through a disordered medium. Strong Rashba
coupling leads to a dramatic reduction of localization effects on the
conductances and their fluctuations, whose features depend on the
spin-polarization of injected electrons. In the limit of weak Rashba
interaction antilocalization vanishes (i.e., the sum of the matrix elements of
${\bf G}$ is almost independent of the SO coupling), but the partial
spin-resolved conductances can still be non-zero. Besides spin-resolved
conductance fluctuations and antilocalization, unusual quantum interference
effects are revealed in this system leading to a negative difference between
the partial conductances for a parallel and an antiparallel orientation of the
contact magnetization, in a range of disorder strengths and for a particular
spin-polarization of incoming electron with respect to the direction of Rashba
electric field.
