Keith,
OK, now I am confused. Are we in agreement? What did I amend?
I used the calculator for skin depth on this page...
http://www.daycounter.com/Calculators/ and then did a quick conversion to mm.
I often point ham radio students to pages like this when discussing RF. It is easy to demo reasons for using copperweld for antennas and to discuss why shields on coaxial cable work.