  We investigate the magnetoresistance of a side-gated ring structure etched
out of single-layer graphene. We observe Aharonov-Bohm oscillations with about
5% visibility. We are able to change the relative phases of the wave functions
in the interfering paths and induce phase jumps of pi in the Aharonov-Bohm
oscillations by changing the voltage applied to the side gate or the back gate.
The observed data can be well interpreted within existing models for 'dirty
metals' giving a phase coherence length of the order of 1 micrometer at a
temperature of 500mK.
