Cut-off frequency in coaxial cable
That formula does not apply to distributed inductances.

Coax cable does have a cutoff frequency, when wavelength of the signal gets somewhere on the order of the diameter of the cable (it's some fraction, like 1/4, but I can't remember exactly).  For most coax, that cutoff frequency is considerably higher than the useful frequency of the cable because of dielectric dissipation -- it is educational to find specifications for various sorts of coax and look at their attenuation/meter at various frequencies.