U.S. shares ended slightly lower in a quiet session Tuesday with trouble at Canada's Nortel Networks the only big attraction. Nortel announced that its chief financial officer, Terry Hungle, had quit after trading company stock in violation of the company rules. The company also lowered its business outlook. The S&P 500 fell 4.44 or 0.4% to end at 1107.50, while the Nasdaq Composite lost 12.45 to 1834.21. The Dow Industrials ended down 21.04 or 0.21% at 9863.74. Dealers said sentiment is cautious though hopes are building that accounting troubles may be passing. They say accounting questions are overshadowing an ever-improving fundamental picture for the market.