  We study theoretically the electrical transport between aligned carbon
nanotubes in nanotube ropes, and between shells in multiwall carbon nanotubes.
We focus on transport between two metallic nanotubes (or shells) of different
chiralities with mismatched Fermi momenta and incommensurate periodicities. We
perform numerical calculations of the transport properties of such systems
within a tight-binding formalism. For clean (disorder-free) nanotubes the
intertube transport is strongly suppressed as a result of momentum
conservation. For clean nanotubes, the intertube transport is typically
dominated by the loss of momentum conservation at the contacts. We discuss in
detail the effects of disorder, which also breaks momentum conservation, and
calculate the effects of localised scatterers of various types. We show that
physically relevant disorder potentials lead to very dramatic enhancements of
the intertube conductance. We show that recent experimental measurements of the
intershell transport in multiwall nanotubes are consistent with our theoretical
results for a model of short-ranged correlated disorder.
