Confirmining Dr. Bergeron's previous input, "Robert W. Stewart" does not appear in Booth's "Records of Louisiana Confederate Soldiers and Louisiana Confederate Commands" (State of Louisiana, New Orleans, 1920). However, Booth's "Records" do not represent an absolutely complete listing of Louisiana Confederate soldiers, and while there are other "on-line" sources of names, your best bet is to contact the National Archives with name, date and place of birth, and place of residence at the beginning of the war. Robert W. Stewart may be simply missing from Booth's "Records" or he could have served with a unit from another state.

Relative to Louisiana units in the Atlanta Campaign, don't forget Gibson's Brigade! And I believe there were also one or two independent Louisiana cavalry companies serving with the headquarters of the Army of Tennessee. I'm thinking of Captain Junius Y. Webb's Squadron of Louisiana Cavalry.

Confederate Army of Tennessee
Stewart's/Clayton's Division
Randall L. Gibson's Louisiana Brigade
1st Louisiana Regulars [Strawbridge's or Batchelor's]
4th Louisiana Infantry regiment
13th Louisiana Infantry regiment
16th/25th Consolidated Louisiana Infantry regiment
19th Louisiana Infantry regiment
20th Louisiana Infantry regiment
30th Louisiana Infantry regiment
4th Battalion of Louisiana Infantry
14th Battalion of Louisiana Sharpshooters [Austin's]

The composition of Randall's Brigade changed during the course of the Atlanta Campaign, but some, if not all of these units were engaged at:
Mill Creek Gap May 7, 1864
Resaca May 14-15, 1864
New Hope Church May 25-27, 1864
Peachtree Creek on July 20, 1864
Ezra Church on July 28, 1864

See Dr. Bergeron's "Guide to Louisiana Confederate Military Units 1861-1865" (LSU Press, 1989) for details on specific regiments, battalions, or companies.