  The two-dimensional layered compound PdCoO$_2$ is one of the best oxide
conductors, providing an intriguing research arena opened by the long mean free
path and the very high mobility of ~51000 cm2/Vs. These properties turn
PdCoO$_2$ into a candidate material for nanoscale quantum devices. By exploring
universal conductance fluctuations originating at nanoscale PdCoO$_2$ Hall-bar
devices, we determined the phase coherence length of electron transport in
c-axis oriented PdCoO$_2$ thin films to equal ~100 nm. The weak temperature
dependence of the measured phase coherence length suggests that defect
scattering at twin boundaries in the PdCoO$_2$ thin film governs phase
breaking. These results suggest that phase coherent devices can be achieved by
realizing the devices smaller than the size of twin domains, via refined
microfabrication and suppression of twin boundaries.
