  We have studied the resistance of single-wall carbon nanotubes measured in a
four-point configuration with noninvasive voltage electrodes. The voltage drop
is detected using multiwalled carbon nanotubes while the current is injected
through nanofabricated Au electrodes. The resistance at room temperature is
shown to be linear with the length as expected for a classical resistor. This
changes at cryogenic temperature; the four-point resistance then depends on the
resistance at the Au-tube interfaces and can even become negative due to
quantum-interference effects.
