  We report on the observation of a non-local voltage in a ballistic
one-dimensional conductor, realized by a single-wall carbon nanotube with four
contacts. The contacts divide the tube into three quantum dots which we control
by the back-gate voltage $V_g$. We measure a large \emph{oscillating} non-local
voltage $V_{nl}$ as a function of $V_g$ with zero mean. Though a classical
resistor model can account for a non-local voltage including change of sign, it
fails to describe the magnitude properly. The large amplitude of $V_{nl}$ is
due to quantum interference effects and can be understood within the
scattering-approach of electron transport.
