Quantum oscillations and low-T resistivity of the 5s delafossite PdCoO$_2$

We report dHvA torque and resistivity data on the highly conductive delafossite PdCoO$_2$. Quantum oscillations confirm electronic structure calculations in showing a single, highly 2D Fermi surface with almost exactly half filling. However the cyclotron masses and the details of the warping are consistent with PdCoO$_2$ being a 5s metal, rather than the 4d metal indicated in most electronic structure papers. The room temperature resistivity is $\rho_{ab}=2.6 \pm 0.2$~$\mu\Omega$-cm, lower than all elemental metals apart from the noble metals. The low-T residual resistivity of our samples is 0.008~$\mu\Omega$-cm, corresponding to an extremely long mean free path of 20~$\mu$m, surprising for a flux-grown material. The temperature dependence of the electron-phonon contribution to the resistivity is exponential rather than $T^5$, indicating phonon drag, and the only observation of phonon drag in resistivity outside the alkali metals.