  The low temperature magnetization process of antiferromagnetic spin-S chains
doped with mobile spin-(S-1/2) carriers is studied in an exactly solvable
model. For sufficiently high magnetic fields the system is in a metallic phase
with a finite gap for magnetic excitations. In this phase which exists for a
large range of carrier concentrations x the zero temperature magnetization is
determined by x alone. This leads to plateaus in the magnetization curve at a
tunable fraction of the saturation magnetization. The critical behaviour at the
edges of these plateaus is studied in detail.
