Federal Rule of Evidence 501 provides that 'the privilege of a witness . . . shall be governed by the principles of the common law as they may be interpreted by the courts of the United States in light of reason and experience.' The attorney-client privilege is the oldest of the privileges for confidential communications known to the common law. J. Wigmore, Evidence § 2290 (McNaughton rev. 1961). Its purpose is to encourage full and frank communication between attorneys and their clients and thereby promote broader public interests in the observance of law and administration of justice. The privilege recognizes that sound legal advice or advocacy serves public ends and that such advice or advocacy depends upon the lawyer's being fully informed by the client. As stated in Trammel v. United States, 445 U.S. 40, 51 (1980): 'The lawyer-client privilege rests on the need for the advocate and counselor to know all that relates to the client's reasons for seeking representation if the professional mission is to be carried out.' And in Fisher v. United States, 425 U.S. 391, 403 (1976), the court recognized the purpose of the privilege to be 'to encourage clients to make full ...