  We report quantitative electrical admittance measurements of diffusive
superconductor--normal-metal--superconductor (SNS) junctions at gigahertz
frequencies and millikelvin temperatures. The gold-palladium-based SNS
junctions are arranged into a chain of superconducting quantum interference
devices. The chain is coupled strongly to a multimode microwave resonator with
a mode spacing of approximately 0.6 GHz. By measuring the resonance frequencies
and quality factors of the resonator modes, we extract the dissipative and
reactive parts of the admittance of the chain. We compare the phase and
temperature dependence of the admittance near 1 GHz to theory based on the
time-dependent Usadel equations. This comparison allows us to identify
important discrepancies between theory and experiment that are not resolved by
including inelastic scattering or elastic spin-flip scattering in the theory.
