  In a telescoped double-walled nanotube (TDWNT), with the inner tube partially
extracted from the outer tube, the total current is forced to flow between the
layers. Considering the interlayer Hamiltonian as a perturbation, we can obtain
an analytic formula for the interlayer conductance. The accuracy of the
perturbation formula is systematically improved by including higher-order
terms. The interlayer interaction effective in the perturbation formula is the
product of the interlayer Hamiltonian and the wave function. It clarifies the
effects of the spatial range of the interlayer Hamiltonian and the band energy
shift.
