By Alina Selyukh WASHINGTON (Reuters) – The top U.S. telecommunications regulator in a letter to a lawmaker on Thursday reasserted his commitment to help smaller national wireless carriers get access to valuable lower-frequency airwaves in the upcoming spectrum auction. Federal Communications Commission Chairman Tom Wheeler reaffirmed his plans to restrict how much spectrum the biggest U.S. carriers, Verizon Communications Inc and ATT Inc, could buy in the auction scheduled for mid-2015. The Incentive Auction offers the opportunity, possibly the last for years to come, to make low-band spectrum available to any mobile wireless provider, in any market, that is willing and able to compete at auction, Wheeler wrote to Representative John Barrow in a letter reviewed by Reuters. The FCC in May will vote on Wheelers proposed auction rules that would reserve part of the spectrum in each market for bidding by wireless carriers that have less than one-third of the low-frequency airwaves, valued for their reach and strength.