(i) Where it is specifically used as a financial term, it should be 'kaikei sekinin', which literally means 'responsibility for providing an explanation for accounting matters'. (ii) Where 'accountability' is used in a more general sense, for example in an expression that goes something like 'to define accountability within an organization', the phrase 'sekinin no shozai', or 「責任の所在」usually fits, which literally means 'the personnel/group of people who are held responsible for'.

Hope this helps.