  We theoretically study the generic behavior of the penetration depth of the
edge states in two-dimensional quantum spin Hall systems. We found that the
momentum-space width of the edge-state dispersion scales with the inverse of
the penetration depth. As an example of well-localized edge states, we take the
Bi(111) ultrathin film. Its edge states are found to extend almost over the
whole Brillouin zone. Correspondingly, the bismuth (111) 1-bilayer system is
proposed to have well-localized edge states in contrast to the HgTe quantum
well.
