  We study conductance fluctuations (CF) and the sensitivity of the conductance
to the motion of a single scatterer in two-dimensional massless Dirac systems.
Our extensive numerical study finds limits to the predicted universal value of
CF. We find that CF are suppressed for ballistic systems near the Dirac point
and approach the universal value at sufficiently strong disorder. The
conductance of massless Dirac fermions is sensitive to the motion of a single
scatterer. CF of order $e^2/h$ result from the motion of a single impurity by a
distance comparable to the Fermi wavelength. This result applies to graphene
systems with a broad range of impurity strength and concentration while the
dependence on the Fermi wavelength can be explored {\em via} gate voltages. Our
prediction can be tested by comparing graphene samples with varying amounts of
disorder and can be used to understand interference effects in mesoscopic
graphene devices.
