  In the one-dimensional Anderson model the eigenstates are localized for
arbitrarily small amounts of disorder. In contrast, the Harper model with its
quasiperiodic potential shows a transition from extended to localized states.
The difference between the two models becomes particularly apparent in phase
space where Heisenberg's uncertainty relation imposes a finite resolution. Our
analysis points to the relevance of the coupling between momentum eigenstates
at weak potential strength for the delocalization of a quantum particle.
