  We study theoretically the impact of Zener tunneling on the charge-transport
properties of quasi-metallic (Qm) carbon nanotubes (characterized by forbidden
band gaps of few tens of meV). We also analyze the interplay between Zener
tunneling and elastic scattering on defects. To this purpose we use a model
based on the master equation for the density matrix, that takes into account
the inter-band Zener transitions induced by the electric field (a quantum
mechanical effect), the electron-defect scattering and the electron-phonon
scattering. In presence of Zener tunnelling the Qm tubes support an electrical
current even when the Fermi energy lies in the forbidden band gap. In absence
of elastic scattering (in high quality samples), the small size of the band gap
of Qm tubes enables Zener tunnelling for realistic values of the the electric
field (above $\sim$ 1 V/\mu m). The presence of a strong elastic scattering (in
low quality samples) further decreases the values of the field required to
observe Zener tunnelling. Indeed, for elastic-scattering lengths of the order
of 50 nm, Zener tunnelling affects the current/voltage characteristic already
in the linear regime. In other words, in quasi-metallic tubes, Zener tunneling
is made more visible by defects.
