From Transmission Lines and Networks by Walter C. Johnson, McGraw-Hill 1963 p58.

Nominal depth of penetration for a conductor 

Where ρ = Is the resistivity of the conductor in Ω/m

f= frequency in Hertz                       

                                    μ = Is the absolute magnetic permeability of the conductor

The absolute magnetic permeability (μ) = μo x μr 

μo  = 4π x 10^-7 H/m

The values for μr are from Transmission Line Design Handbook by Brian C Wadell, Artech House 1991 Table 9.3.2 page 446.

The values for ρ are from CRC Handbook of Chemistry and Physics 1st Student Edition 1998 page F-88 and are for high purity elements at 20°C.