[Nonfractal]

Jeremy,
Carbon rear axles are no use at all. 
The Carbon is structurally strong along its length but is prone to twist causing the car to weave and  "Swerve"  out if the slot. 
Even using precision bearings (Carbon just doesn't run smoothly in bronze bushes) I never got these to work. 

I use titanium slotingplus rear axles but am starting to move toward steel hollow instead after I bent a titanium Axle.  BENT!  Much softer than I expected. 
I can wear the extra 0.75 grams for a steel hollow axle.

I doubt that "magnetic drag" between the motor and axle is a real thing. If the motor's magnets really were able to induce eddy currents in the axle, creating a drag force, the Levitron device wouldn't work.
     

[BAracer]

My best bike frame is made out of titanium by these wonderful people who are just five miles from me...

Enigma Bikes

They don't use ti because of its strength per se, they use it because it is soft and absorbs road 'buzz'.

[Kevan]

I think you'll find they use Ti because it's strong enough but mainly because it'so lightweight.

[Brumos RSR]

I’ve built 2 podium proxy cars that have used carbon fiber axles with bearings.  Steel is my primary but it can be done if not over powered. Sorry, I guess I should or should not have shared a proxy secret of mine. Back to observation mode.

[chrisguyw]

I use titanium slotingplus rear axles but am starting to move toward steel hollow instead after I bent a titanium Axle.  BENT!  Much softer than I expected. 

 

If you are set on hollow steel axles, try the ones from Koford,.......KOF-M339, KOF-340,.........these are top quality/precision products, much better than the ones from the plastic car manufacturers.

Cheers
Chris Walker