  We present a low energy-theory for non-linear transport in finite-size
interacting single-wall carbon nanotubes. It is based on a microscopic model
for the interacting p_z electrons and successive bosonization. We consider weak
coupling to the leads and derive equations of motion for the reduced density
matrix. We focus on the case of large-diameter nanotubes where exchange effects
can be neglected. In this situation the energy spectrum is highly degenerate.
Due to the multiple degeneracy, diagonal as well as off-diagonal (coherences)
elements of the density matrix contribute to the nonlinear transport. At low
bias, a four-electron periodicity with a characteristic ratio between adjacent
peaks is predicted. Our results are in quantitative agreement with recent
experiments.
