modes of the structure even though its thickness is comparable with that of the SiN$_{\rm x}$. The superconducting wiring also contributes negligible electronic heat conduction because the quasiparticle density is exceedingly small at low temperatures. Figure \[fig:TES\_Images\](h) shows a Scanning Electron Micrograph of a 1R phononic leg, viewed obliquely.

Figure \[fig:TES\_Images\] illustrates that it is possible to fabricate few-mode multi-element interferometers, with 500nm wide features, outstanding definition, and well-aligned Nb wiring. It is remarkable that these tiny patterned legs are perfectly able to support the main body of the TES, and can be fabricated with high yield, which was due in part to our ability to control film stresses in the main body of the device. As will be seen later, it is also notable that these devices performed perfectly well as TESs, with no evidence of anomalous behaviour, such as weak links or additional stray resistance where the Nb leads meandered over the arms of the interferometers. As will be seen later, the thermal properties of these tiny structures were fully consistent with few-mode elastic behaviour even though they were supporting the relatively large central island of the TES. This occurs because the bulk elastic constants are relatively insensitive 