  Inversion asymmetry induced spin splitting of the electron states in quasi
two-dimensional (2D) systems can be attributed to an effective magnetic field B
which varies in magnitude and orientation as a function of the in-plane wave
vector k||. Using a realistic 8x8 Kane model that fully takes into account spin
splitting because of both bulk inversion asymmetry and structure inversion
asymmetry we investigate the spin orientation and the effective field B for
different configurations of a quasi 2D electron system. It is shown that these
quantities depend sensitively on the crystallographic direction in which the
quasi 2D system was grown as well as on the magnitude and orientation of the
in-plane wave vector k||. These results are used to discuss how spin-polarized
electrons can precess in the field B(k||). As a specific example we consider
GaInAs-InP quantum wells.
