Steve Bannon is now under subpoena by both House investigators and the Special Counsel after he refused to answer questions before Congress. Following a problematic pattern of current and former Administration figures, Bannon reportedly did not invoke executive privilege (which must be asserted by the White House) but refused to answer questions about his work on the transition team and White House. There is no basis for such a broad assertion of executive privilege and, unless Bannon changes course, he could be looking at contempt sanctions down the road.
Reports indicate that the appearance of Bannon before House Intelligence Committee became heated and chaotic as Bannon refused to speak about his transition or White House service. This could be a play by his counsel for some form of immunity or a deal narrowing the questions. However, reports indicate that the White House may have gagged Bannon.
The use of executive privilege over conversations with a president-elect is very hard to maintain. Trump was not the chief executive until he took the oath of office.