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.