  We describe the spin-dependent quantum conductance in a wire where a magnetic
field is spatially modulated. The change in direction and intensity of the
magnetic field acts as a perturbation that mixes spin projections. This is
exemplified by a ferromagnetic nanowire. There the local field varies smoothly
its direction generating a domain wall (DW) as described by the well-known
Cabrera-Falicov model. Here, we generalize this model to include also a
strength modulation. We identify two striking diabatic regimes that appear when
such magnetic inhogeneity occurs. 1) If the field strength at the DW is weak
enough, the local Zeeman energies result in an avoided crossing. Thus, the
spin-flip probability follows the Landau-Zener formula. 2) For strong fields,
the spin-dependent conductance shows oscillations as a function of the DW
width. We interpret them in terms of Rabi oscillations. Time and length scales
obtained from this simplified view show an excellent agreement with the exact
dynamical solution of the spin-dependent transport. These results remain valid
for other situations involving modulated magnetic structures and thus they open
new prospects for the use of quantum interferences in spin-based devices.
  This paper is dedicated to the memory of the lifelong collaborator Patricia
Rebeca Levstein.
