  Sharp localization transitions of chiral edge states in disordered quantum
wires, subject to strong magnetic field, are shown to be driven by crossovers
from two- to one-dimensional localization of bulk states. As a result, the
two-terminal conductance is found to exhibit at zero temperature discontinuous
transitions between {\it exactly} integer plateau values and zero, reminiscent
of first order phase transitions. We discuss the corresponding phase diagram.
The spin of the electrons is shown to result in a multitude of phases, when the
spin degeneracy is lifted by the Zeeman energy. The width of conductance
plateaus is found to depend sensitively on the spin flip rate $1/\tau_s$.
