  The influence of dephasing on the quantum spin Hall effect (QSHE) is studied.
In the absence of dephasing, the longitudinal resistance in a QSHE system
exhibits the quantum plateaus. We find that these quantum plateaus are robust
against the normal dephasing but fragile with the spin dephasing. Thus, these
quantum plateaus only survive in mesoscopic samples. Moreover, the longitudinal
resistance increases linearly with the sample length but is insensitive to the
sample width. These characters are in excellent agreement with the recent
experimental results [science {\bf 318}, 766 (2007)]. In addition, we define a
new spin Hall resistance that also exhibits quantum plateaus. In particular,
these plateaus are robust against any type of dephasing and therefore, survive
in macroscopic samples and better reflect the topological nature of QSHE.
