The use of ellipses to show trailing off has gained favor in many segments of the court reporting community. Many reporters express a desire to distinguish between a speaker who trails off and a speaker who is interrupted.
The dash in English shows broken sentence structure. It does not matter how the structure gets broken. It simply shows that a sentence did not get finished. Its use is not restricted to just an interruption. The dash to show that a sentence did not get finished is ALWAYS correct.
Though in English ellipses are generally reserved for indicating that something has been left out that was included in the original, usually used inside of quotes, their use showing trailing off is supported by Merriam-Webster’s Eleventh by the following: “mark or marks indicating an omission or a pause.”
There was a recent case in court, reported by a local court reporter, in which the judge admonished an attorney for the number of times he interrupted the witness during a prior deposition in the case. This would certainly give justification for making a distinction between being interrupted and trailing off.
Other options for trailing off — such as a dash followed by a period, a dash with the space in front of it omitted, et cetera — are to be avoided.