  We study the microwave impedance of extremely high aspect ratio (length/width
~ 5,000) superconducting niobium nitride nanowires. The nanowires are
fabricated in a compact meander geometry that is in series with the center
conductor of a 50 ohm coplanar waveguide transmission line. The transmission
coefficient of the sample is measured up to 20 GHz. At high frequency, a peak
in the transmission coefficient is seen. Numerical simulations show that this
is a half-wave resonance along the length of the nanowire, where the nanowire
acts as a high impedance, slow wave transmission line. This resonance sets the
upper frequency limit for these nanowires as inductive elements. Fitting
simulations to the measured resonance enables a precise determination of the
nanowire's complex sheet impedance at the resonance frequency. The real part is
a measure of dissipation, while the imaginary part is dominated by kinetic
inductance. We characterize the dependence of the sheet resistance and sheet
inductance on both temperature and current and compare the results to recent
theoretical predictions for disordered superconductors. These results can aid
in the understanding of high frequency devices based on superconducting
nanowires. They may also lead to the development of novel superconducting
devices such as ultra-compact resonators and slow-wave structures.
