Some unknown material has a conductivity of $10^{6} \; \mathrm{mho} / \mathrm{m}$ and a permeability of $4 \pi \times 10^{-7} \; \mathrm{H} / \mathrm{m}$. The skin depth for the material at $1 \; \mathrm{GHz}$ is
- $15.9 \; \mu \mathrm{m}$
- $20.9 \; \mu \mathrm{m}$
- $25.9 \; \mu \mathrm{m}$
- $30.9 \; \mu \mathrm{m}$