  We advert to the fact that presence of valence fluctuations (VFs) in
semiconductors with in-gap impurity bands unconditionally leads to dynamical
changes (fluctuations) of energies of localized impurity states. We provide
arguments that in the impurity subnetwork consisting of centers having energy
levels fluctuating around the Fermi energy there exist favorable conditions for
hops from occupied states to empty states of less energy. Consequently, we
propose original valence-fluctuation induced hopping mechanism as a new
possibility to explain unusual metallic-like conduction of SmB$_6$ and other
Kondo insulators experimentally observed at lowest temperatures. Interestingly,
the proposed mechanism infers enhanced metallic-like surface conductivity of
SmB$_6$, what resembles a characteristic property of topological insulator, and
is in agreement with experimental observations attempting to prove the
existence of topologically protected surface state in SmB$_6$.
