A 64-year-old man presents with symptoms of difficulty swallowing and weight loss of 10 lb. He has no prior history of heartburn, stomach ulcers, or difficulty swallowing. He smokes one pack a day for the past 45 years and drinks approximately 5 oz of alcohol a day. He is thin appearing, there are no oral lesions, and the remaining examination is normal. Esophagoscopy reveals a midesophageal narrowing with ragged ulcerating, and biopsies are taken. Which of the following is the most likely diagnosis?