The scanners at McCarran are not Backscatters. They are Millimeter wave. Backscatters can be identified by two large black parallel monoliths. MMW's are the glass hexagonal or octagonal scanners like McCarran has. The only place I have encountered Backscatters is Boston.
A list of airports and what they have (and how to avoid them) can be found at
https://www.flyertalk.com/forum/practical-travel-safety-issues/1138014-complete-list-airports-whole-body-imaging-advanced-imaging-technology-scanner.html
- bcc