  We show that wavefunctions in a two-dimensional (2D) electron system with
spin-orbit coupling can be characterized by a topological quantity--the Chern
integer due to the existence of the intrinsic Kramers degeneracy. The
localization-delocalization transition in such a system is studied in terms of
such a Chern number description, which reproduces the known metal-insulator
transition point. The present work suggests a unified picture for various known
2D delocalization phenomena based on the same topological characterization.
