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 quasibound 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.