As the Federal Communications Commission's spectrum auction nears closure, Sprint Corp. and T-Mobile US Inc. soon will have an opening for merger talks. Though T-Mobile (Nasdaq: TMUS) and other participants are still in a "quiet period" until the auction ends on March 30, both carriers have hinted at a possible deal. Sprint CEO Marcelo Claure has said the Overland Park-based company (NYSE: S) would be a " formidable partner to potentially merge with another carrier," without specifying the owner. SoftBank…