  We report here on electrical measurements on individual multi-walled carbon
nanotubes (MWNTs) that show that the presence or movement of impurities or
defects in the carbon nanotube can radically change its low temperature
transport characteristics. The low temperature conductance can either decrease
monotonically with decreasing temperature, or show a sudden increase at very
low temperatures, sometimes in the same sample at different times. This unusual
behavior of the temperature dependence of the conductance is correlated with
large variations in the differential conductance as a function of the dc
voltage across the wire. The effect is well described as arising from quantum
interference of conduction channels corresponding to direct transmission
through the nanotube and resonant transmission through a discrete electron
state, the so-called Fano resonance.
