  By studying the temperature-dependent behavior of electron thermal
conductivity (k) in a 3.2 nm-thin film, we quantify the extremely confined
defect-electron scattering and reveal the intrinsic phonon-electron scattering
that is shared by bulk Ir. At low temperatures below 50 K, the thermal
conductivity of the thin film has almost two orders of magnitude reduction from
that of the bulk Ir. The thermal conductivity of the film increases with
increasing temperature while that of the bulk Ir has an opposite trend. We
introduce a unified thermal resistivity to interpret this completely different
k-T relation. This residual thermal resistivity provides an unprecedented way
to quantitatively evaluating defect-electron scatterings in heat conduction.
The interfacial thermal conductance across the grain boundaries is found larger
than that of the Al/Cu interface. Its value is proportional to temperature
largely because of the electron's specific heat. A unified interfacial thermal
conductance is defined and firmly proves such relation. The electron reflection
coefficient is found to be large (88%) and almost temperature independent. This
means most of the electrons which scatter with the grain boundary would be
reflected back and the scatterings are not affected by temperature.
