  The Dorokhov-Mello-Pereyra-Kumar (DMPK) equation for transmission eigenvalues
is derived for metallic carbon nanotubes with several conducting channels when
the potential range of scatterers is larger than the lattice constant. With
increasing system length L, the system approaches a fixed point, where only one
channel is perfectly conducting and other channels are completely closed. The
asymptotic behavior of the conductance in the long-L regime is investigated on
the basis of the DMPK equation. It is shown that the length scale for the
exponential decay of the typical conductance is reduced due to the presence of
the perfectly conducting channel. If a magnetic field is applied, the system
falls into the unitary class. It is pointed out that this transition is
characterized by the disappearance of the perfectly conducting channel and the
increase in decay length for the typical conductance.
