  Electron-electron collisions lead to a T-square component in the electrical
resistivity of Fermi liquids. The case of liquid $^3$He illustrates that the
\textit{thermal} resitivity of a Fermi liquid has a T-square term, expressed in
m$\cdot$W$^{-1}$. Its natural units are $\hbar/k_FE_F^2$. Here, we present a
high-resolution study of the thermal conductivity in bismuth, employing
magnetic field to extract the tiny electronic component of the total thermal
conductivity and resolving signals as small as $\approx 60 \mu$K. We find that
the electronic thermal resistivity follows a T-square temperature dependence
with a prefactor twice larger than the electric T-square prefactor. Adding this
information to what has been known for other semi-metals, we find that the
prefactor of the T-square thermal resistivity scales with the square of the
inverse of the Fermi temperature, implying that the dimensionless
fermion-fermion collision cross-section is roughly proportional to the Fermi
wavelength, indicating that it is not simply set by the strength of the Coulomb
interaction.
