  The low temperature acoustic properties of bulk metallic glasses measured
over a broad range of frequencies rigorously test the predictions of the
standard tunneling model. The strength of these experiments and their analyses
is mainly based on the interaction of the tunneling states with conduction
electrons or quasiparticles in the superconducting state. A new series of
experiments at kHz and GHz frequencies on the same sample material essentially
confirms previous measurements and their discrepancies with theoretical
predictions. These discrepancies can be lifted by considering more correctly
the line widths of the dominating two-level atomic-tunneling systems. In fact,
dephasing caused or mediated by interaction with conduction electrons may lead
to particularly large line widths and destroy the tunneling sytems' two-level
character in the normal conducting state.
