Long-term U.S. mortgage rates are down this week amid a restrained home buying season this summer. Mortgage buyer Freddie Mac says the average rate on 30-year, fixed-rate mortgages slipped to 4.59 percent from 4.60 percent last week. Long-term loan rates have been running at their highest levels in seven years. The average benchmark 30-year rate reached a high this year of 4.66 percent on May 24. By contrast, the rate stood at 3.90 percent a year ago.