We discuss zero-temperature quantum spin chains in a uniform magnetic field,
with axial symmetry. For integer or half-integer spin, S, the magnetization
curve can have plateaus and we argue that the magnetization per site m is
topologically quantized as q(Sām)=integer at the plateaus, where q is
the period of the groundstate. We also discuss conditions for the presence of
the plateau at those quantized values. For S=3/2 and m=1/2, we study
several models and find two distinct types of massive phases at the plateau.
One of them is argued to be a ``Haldane gap phase'' for half-integer S.Comment: Revised version, to appear in Phys. Rev. Lett. (no changes in main
conclusions); 5 pages, REVTEX with 2 figures in ep