  The disorder-driven metal-insulator transition in the quantum spin Hall
systems is studied by scaling analysis of the Thouless conductance $g$. Below a
critical disorder strength, the conductance is independent of the sample size
$M$, an indication of critically delocalized electron states. The calculated
beta function $\beta=d\ln g/d\ln M$ indicates that the metal-insulator
transition is Kosterlitz-Thouless (KT) type, which is characterized by bounding
and unbounding of vortex-antivortex pairs of the local currents. The KT like
metal-insulator transition is a basic characteristic of the quantum spin Hall
state, being independent of the time-reversal symmetry.
