The average long-term U.S. mortgage rate tumbled by nearly a half-point this week, but will likely remain a significant barrier for potential homebuyers as Federal Reserve officials have all but promised more rate hikes in the coming months. Mortgage buyer Freddie Mac reported Thursday that the average on the key 30-year rate fell to 6.61% from 7.08% last week. Three weeks ago, the average long-term U.S. mortgage rate topped 7% for the first time in more than two decades, which combined with sky-high home prices, have crushed homebuyers’ purchasing power by adding hundreds of dollars to monthly mortgage payments.