Possible explanation for the conductance of a single quantum unit in metallic carbon nanotubes

The quantum conductance of a metallic carbon nanotube with one end immersed in a jellium metal is studied. We find that the incident pi*-band electron s, having a very high angular momentum with respect to the tube axis, go th rough the tube without being scattered by the free electrons in surrounding metal and contribute a quantum unit (2e(2)/h) to the conductance. On the o ther hand, the incident pi-band electrons, with the p(z) atomic orbitals in phase along the tube circumference, experience strong resonant back-scatte ring because the low-angular-momentum states at the Fermi level have a domi nantly metallic character in the nanotube-jellium metal coexistence region. These results provide a possible explanation for the experimentally observ ed conductance of one quantum unit instead of two for nanotubes with one en d dipped into liquid metal such as mercury. [S0163-1829(99)51044-8].