  We report a study of transport blockade features in a quantum dot
single-electron transistor, based on an undoped AlGaAs/GaAs heterostructure. We
observe suppression of transport through the ground state of the dot, as well
as negative differential conductance at finite source-drain bias. The
temperature and magnetic field dependence of these features indicate the
couplings between the leads and the quantum dot states are suppressed. We
attribute this to two possible mechanisms: spin effects which determine whether
a particular charge transition is allowed based on the change in total spin,
and the interference effects that arise from coherent tunneling of electrons in
the dot.
