  In mesoscopic physics, interference effects play a central role on the
transport properties of conduction electrons, giving rise to exotic phenomena
such as weak localization, Aharonov-Bohm effect, and universal conduction
fluctuations. Mesoscopic objects have a size on the order of the {\em
phase-breaking length} $L_{\phi}$, the length conduction electrons travel while
keeping phase coherence. In this letter, we use vibrational spectroscopy in
combination with a novel optical defocusing method to measure $L_{\phi}$ of
photo-excited electrons in graphene which undergo inelastic scattering by
optical phonons. We extract $L_{\phi}$ from the spatial confinement of the
defect-induced Raman D band near the edges of graphene. Temperature dependent
measurements in the range of 1.55\,K to 300\,K yield $L_{\phi} \propto
1/\sqrt{T}$, in agreement with previous magneto-transport measurements.
