Semirigid coaxial lines working up to 50-60 GHz are convenient for low temperature experiments for two main reasons:

1) They are thin, so they do not transfer too much heat down to the cryostat, allowing to work at temperatures down to 15 mK.

2) They do not have a cut-off frequency, as waveguides do. This means that coaxial lines can be used from 0 to 50 GHz without problems. However, the energy losses become important at high freqeuencies.

waveguides

Waveguides have several advantages over two-wire and coaxial transmission lines. For example, the large surface area of waveguides greatly reduces copper (I²R) losses. Two-wire transmission lines have large copper losses because they have a relatively small surface area. The surface area of the outer conductor of a coaxial cable is large, but the surface area of the inner conductor is relatively small. At microwave frequencies, the current-carrying area of the inner conductor is restricted to a very small layer at the surface of the conductor by an action called skin effect. Skin effect tends to increase the effective resistance of the conductor.