[@DJBallistic]. The thermal conductance and thermal fluctuation noise in these devices was found to be fully predicted by heat transport calculations based solely on the dispersion curves of elastic modes calculated using the bulk elastic constants of the material. Moreover, the uniformity in performance was high as a consequence of having eliminated resonant phonon scattering in the disorder of the material.

At low temperatures ($\leq$ 150mK), heat is transported in low-dimensional dielectric bars through a small number of elastic modes. In our ballistic devices [@DJBallistic], approximately 6-7 modes were excited, which is close to the quantised limit of 4: one compressional, one torsional, and in-plane and out-of-plane flexure. In a subsequent series of experiments [@ThermalAttenuation2017], we measured the thermal elastic attenuation length of these modes to be 20$\mu$m, and so our short-legged TESs were operating well within the ballistic limit. It can be shown, and was found in practice, that the ballistic, few mode limit corresponds to an NEP of approximately 10$^{-18}$WHz$^{-1/2}$. This NEP cannot be reduced further by increasing the length, because there is no scattering, or reducing the cross section, because we have already reached the quantised limit. The question arises as to whether it is possible to 