  The isolation of energetically persistent scattering pathways from the
resonant manifold of an open electron billiard in the deep quantum regime is
demonstrated. This enables efficient conductance switching at varying
temperature and Fermi velocity, using a weak magnetic field. The effect relies
on the interplay between magnetic focusing and soft-wall confinement, which
rescale the scattering pathways and decouple quasi-bound states from the
attached leads, the field-free motion being forwardly collimated. The mechanism
proves robust against billiard shape variations and qualifies as a
nanoelectronic current control element.
