  We report microwave surface impedances of FeSe$_{0.4}$Te$_{0.6}$ single
crystals measured at 12, 19, and 44 GHz. The penetration depth exhibits a power
law behavior, $\delta \lambda_L=\lambda_L (T)-\lambda_L (0) \propto CT^n$ with
an exponent $n\simeq 2$, which is considered to result from impurity
scattering. This behavior is consistent with $s\pm$-wave pairing symmetry. The
temperature dependence of the superfluid density largely deviates from the
behavior expected in the BCS theory. We believe that this deviation is caused
by the crossover from the dirty regime near $T_c$ to the clean regime at low
temperatures, which is supported by the rapid increase of the quasiparticle
scattering time obtained from the microwave conductivity. We also believe that
the previously published data of the superfluid density can be interpreted in
this scenario.
