  Charge and spin density waves are highly correlated electron systems that can
transport an electric current. A model is discussed in which a field E induces
the creation, by quantum tunneling, of pairs of oppositely charged solitons and
antisolitons in a density wave. Pair creation is blocked, for fields below a
threshold value ET, because the increase in electrostatic energy would violate
energy conservation. When E > ET, this Coulomb blockade mechanism time-
correlates the pair creation and annihilation events. The model provides a
natural explanation for the extremely small phase displacements below threshold
suggested by NMR and other experiments.
