  The dimensional crossover in a spin-$S$ nearest neighbor Heisenberg
antiferromagnet is discussed as it is tuned from a two-dimensional square
lattice, of lattice spacing $a$, towards a spin chain by varying the width
$L_y$ of a semi-infinite strip $L_x\times L_y$. For integer spins and arbitrary
$L_y$, and for half integer spins with $L_y/a$ an arbitrary even integer,
explicit analytical expressions for the zero temperature correlation length and
the spin gap are given. For half integer spins and $L_y/a$ an odd inetger, it
is shown that the $c=1$ behavior of the $SU(2)_1$ WZW fixed point is squeezed
out as the width $L_y\to \infty$; here $c$ is the conformal charge. The results
specialized to $S=1/2$ are relevant to spin-ladder systems.
