  Ohm's law describes the proportionality of current density and electric
field. In solid-state conductors, Ohm's law emerges due to electron scattering
processes that relax the electrical current. Here, we use nitrogen-vacancy
center magnetometry to directly image the local breakdown of Ohm's law in a
narrow constriction fabricated in a high mobility graphene monolayer. Ohmic
flow is visible at room temperature as current concentration on the
constriction edges, with flow profiles entirely determined by sample geometry.
However, as the temperature is lowered below 200 K, the current concentrates
near the constriction center. The change in the flow pattern is consistent with
a crossover from diffusive to viscous electron transport dominated by
electron-electron scattering processes that do not relax current.
