Name the broadcast date for 6.4 million viewers for the archive of 16mm t/r
CREATE TABLE "table1_1776943_1" (
  "episode" text,
  "broadcast_date" text,
  "run_time" text,
  "viewers_in_millions" text,
  "archive" text
);
SELECT "broadcast_date" FROM "table1_1776943_1" WHERE "viewers_in_millions"='6.4' AND "archive"='16mm t/r';