We have experimentally investigated quantum interference corrections to theconductivity of graphene nanoribbons at temperatures down to 20 mK studyingboth weak localization (WL) and universal conductance fluctuations (UCF). Sincein individual nanoribbons at millikelvin temperatures the UCFs strongly maskthe weak localization feature we employ both gate averaging and ensembleaveraging to suppress the UCFs. This allows us to extract the phase coherencelength from both WL and UCF at all temperatures. Above 1 K, the phase coherencelength is suppressed due to Nyquist scattering whereas at low temperatures weobserve a saturation of the phase coherence length at a few hundred nanometers,which exceeds the ribbon width, but stays below values typically found in bulkgraphene. To better describe the experiments at elevated temperatures, weextend the formula for 1D weak localization in graphene, which was derived inthe limit of strong intervalley scattering, to include all elastic scatteringrates.
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