We have experimentally investigated quantum interference corrections to the
conductivity of graphene nanoribbons at temperatures down to 20 mK studying
both weak localization (WL) and universal conductance fluctuations (UCF). Since
in individual nanoribbons at millikelvin temperatures the UCFs strongly mask
the weak localization feature we employ both gate averaging and ensemble
averaging to suppress the UCFs. This allows us to extract the phase coherence
length from both WL and UCF at all temperatures. Above 1 K, the phase coherence
length is suppressed due to Nyquist scattering whereas at low temperatures we
observe a saturation of the phase coherence length at a few hundred nanometers,
which exceeds the ribbon width, but stays below values typically found in bulk
graphene. To better describe the experiments at elevated temperatures, we
extend the formula for 1D weak localization in graphene, which was derived in
the limit of strong intervalley scattering, to include all elastic scattering
rates.Comment: 8 pages, 6 figures, accepted by PR