  We report thorough measurements of surface plasmon polaritons (SPPs) running
along nearly perfect air-gold interfaces formed by atomically flat surfaces of
chemically synthesized gold monocrystals. By means of amplitude- and
phase-resolved near-field microscopy, we obtain their propagation length and
effective mode index at visible wavelengths (532, 594, 632.8, 729, and 800 nm).
The measured values are compared with the values obtained from the dielectric
functions of gold that are reported in literature. Importantly, a reported
dielectric function of monocrystalline gold implies $\sim 1.5$ times shorter
propagation lengths than those observed in our experiments, whereas a
dielectric function reported for properly fabricated polycrystalline gold leads
to SPP propagation lengths matching our results. We argue that the SPP
propagation lengths measured in our experiments signify the ultimate limit of
optical losses in gold, encouraging further comprehensive characterization of
optical material properties of pure gold as well as other plasmonic materials.
