  We explore the quantum transmission through open oval shaped quantum dots.
The transmission spectra show periodic resonances and, depending on the
geometry parameter, a strong suppression of the transmission for low energies.
Applying a weak perpendicular magnetic field changes this situation drastically
and introduces a large conductance. We identify the underlying mechanisms being
partially due to the specific shape of the oval that causes a systematic
decoupling of a substantial number of states from the leads. Importantly a
pairwise destructive interference of the transmitting states is encountered
thereby leading to the complete conductance suppression. Coupling properties
and interferences can be tuned via a weak magnetic field. These properties are
robust with respect to the presence of disorder in the quantum dot.
