  We measured the microwave surface impedance of FeSe$_{0.4}$Te$_{0.6}$ single
crystals with- and without external magnetic fields. The superfluid density
exhibited a quadratic temperature dependence, indicating a strong pair-breaking
effect. The flux-flow resistivity behaved as $\rho_f(B\ll B_{\rm
c2})/\rho_n=\alpha B/B_{\rm c2}$. The observed $\alpha$ value of $\approx0.66$
was considerably smaller than that of other Fe-based materials ($\alpha\geq1$)
and was attributed to a back-flow of superfluids remarkable in disordered
superconductors. This is the first-time observation of the back-flow phenomenon
caused by an origin other than the vortex pinning in multiple-band systems.
