  Quantum mechanics and Coulomb interaction dictate the behavior of small
circuits. The thermal implications cover fundamental topics from quantum
control of heat to quantum thermodynamics, with prospects of novel thermal
machines and an ineluctably growing influence on nanocircuit engineering.
Experimentally, the rare observations thus far include the universal thermal
conductance quantum and heat interferometry. However, evidences for many-body
thermal effects paving the way to markedly different heat and electrical
behaviors in quantum circuits remain wanting. Here we report on the observation
of the Coulomb blockade of electronic heat flow from a small metallic circuit
node, beyond the widespread Wiedemann-Franz law paradigm. We demonstrate this
thermal many-body phenomenon for perfect (ballistic) conduction channels to the
node, where it amounts to the universal suppression of precisely one quantum of
conductance for the transport of heat, but none for electricity. The
inter-channel correlations that give rise to such selective heat current
reduction emerge from local charge conservation, in the floating node over the
full thermal frequency range ($\lesssim$temperature$\times k_\mathrm{B}/h$).
This observation establishes the different nature of the quantum laws for
thermal transport in nanocircuits.
