Can I span conduit 15 feet between my home and unattached garage?
You could also fabricate a "strut"* structure to span the distance, then strap the conduit to the strut. The issue is that you can't rely on the mechanical strength of the conduit to support itself (and the wire inside of it) for distances over 10ft.

*(Unitstrut or Superstrut are brands of construction members for making light frameworks for holding pipes and conduits typically sold at plumbing and electrical supply houses, but sometimes available at larger home centers) 

http://www.tnb.com/pub/en/brands/superstrut

https://www.unistrut.us/