  A low energy-theory for non-linear transport in finite-size single-wall
carbon nanotubes, based on a microscopic model for the interacting pz electrons
and successive bosonization, is presented. Due to the multiple degeneracy of
the energy spectrum diagonal as well as off-diagonal (coherences) elements of
the reduced density matrix contribute to the nonlinear transport. A
four-electron periodicity with a characteristic ratio between adjacent peaks,
as well as nonlinear transport features, in quantitative agreement with recent
experiments, are predicted.
