  A controlled decoherence environment is studied experimentally by free
electron interaction with semiconducting and metallic plates. The results are
compared with physical models based on decoherence theory to investigate the
quantum-classical transition. The experiment is consistent with decoherence
theory and rules out established Coulomb interaction models in favor of
plasmonic excitation models. In contrast to previous decoherence experiments,
the present experiment is sensitive to the onset of decoherence.
