  Graphene has opened new avenues of research in quantum transport, with
potential applications for coherent electronics. Coherent transport depends
sensitively on scattering from microscopic disorder present in graphene
samples: electron waves traveling along different paths interfere, changing the
total conductance. Weak localization is produced by the coherent backscattering
of waves, while universal conductance fluctuations are created by summing over
all paths. In this work, we obtain conductance images of weak localization with
a liquid-He-cooled scanning probe microscope, by using the tip to create a
movable scatterer in a graphene device. This technique allows us to investigate
coherent transport with a probe of size comparable to the electron wavelength.
Images of magnetoconductance \textit{vs.} tip position map the effects of
disorder by moving a single scatterer, revealing how electron interference is
modified by the tip perturbation. The weak localization dip in conductivity at
B=0 is obtained by averaging magnetoconductance traces at different positions
of the tip-created scatterer. The width $\Delta B_{WL}$ of the dip yields an
estimate of the electron coherence length $L_\phi$ at fixed charge density.
This "scanning scatterer" method provides a new way of investigating coherent
transport in graphene by directly perturbing the disorder configuration that
creates these interferometric effects.
