An efficient way is developed to calculate the matrix elements of Green's functions in real space, by which conductances of the long clean incommensurate double-walled carbon nanotubes (DWNTs) are calculated. It is shown that, in a certain lower energy region, the conductances keep (2)G(0) except for at a few energy positions for the system length at least up to about 1.4 mum, and when the tube length increasing to be 10 mum order, the conductance tend to be 1G(0), which is in good consistency with the experiments. The electron motion in the other higher energy regions becomes nonballistic. (C) 2004 Elsevier B.V. All rights reserved.