  Silver is considered to be the king among plasmonic materials because it
features low inelastic absorption in the visible and infrared (vis-IR) spectral
regions compared to other metals. In contrast, copper is commonly regarded as
being too lossy for plasmonic applications. Here, we experimentally demonstrate
vis-IR plasmons in long copper nanowires (NWs) with quality factors that exceed
a value of 60, as determined by spatially resolved, high-resolution electron
energy-loss spectroscopy (EELS) measurements. We explain this counterintuitive
result by the fact that plasmons in these metal wires have most of their
electromagnetic energy outside the metal, and thus, they are less sensitive to
inelastic losses in the material. We present an extensive set of data acquired
on long silver and copper NWs of varying diameters supporting this conclusion
and further allowing us to understand the relative roles played by radiative
and nonradiative losses in plasmons that span a wide range of energies down to
$<20\,$meV. At such small plasmon energies, thermal population of these modes
becomes significant enough to enable the observation of electron energy gains
associated with plasmon absorption events. Our results support the use of
copper as an attractive cheap and abundant material platform for high quality
plasmons in elongated nanostructures.
