  The Wiedemann-Franz law states that the charge conductance and the electronic
contribution to the heat conductance are proportional. This sets stringent
constraints on efficiency bounds for thermoelectric applications, which seek
for large charge conduction in response to a small heat flow. We present
experiments based on a quantum dot formed inside a semiconducting InAs nanowire
transistor, in which the heat conduction can be tuned significantly below the
Wiedemann-Franz prediction. Comparison with scattering theory shows that this
is caused by quantum confinement and the resulting energy-selective transport
properties of the quantum dot. Our results open up perspectives for tailoring
independently the heat and electrical conduction properties in semiconductor
nanostructures.
