  In two-dimensional (2D) hole systems the inversion asymmetry induced spin
splitting differs remarkably from its familiar counterpart in the conduction
band. While the so-called Rashba spin splitting of electron states increases
linearly with in-plane wave vector k_|| the spin splitting of heavy hole states
can be of third order in k_|| so that spin splitting becomes negligible in the
limit of small 2D hole densities. We discuss consequences of this behavior in
the context of recent arguments on the origin of the metal-insulator transition
observed in 2D systems.
