  In quasi two-dimensional (2D) hole systems with an effective spin j=3/2,
heavy hole-light hole splitting results in a quantization of angular momentum
perpendicular to the 2D plane. The spin polarization of quasi 2D hole systems
due to an in-plane magnetic field B thus competes with the heavy hole-light
hole splitting. As a result, the spin polarization of hole systems is very
different from the spin polarization of j=1/2 electron systems. In particular,
it is shown that the spin polarization of quasi 2D heavy hole systems can
change its sign at a finite value of B.
