  We present first measurements of quantum oscillations in the layered oxide
superconductor
  Ag5Pb2O6. From a detailed angular and temperature dependent study of the dHvA
effect we determine the electronic structure and demonstrate that the electron
masses are very light, m^* is approximately equalt to 1.2 m_e. The Fermi
surface we observe is essentially that expected of nearly-free electrons -
establishing
  Ag5Pb2O6 as the first known example of a monovalent, nearly-free electron
superconductor at ambient pressure.
